Activation of fructose-1,6-bisphosphatases by monovalent cations and its relationship with a fructose-2,6-bisphosphate allosteric site

Abstract

The effects of potassium ions on pig kidney fructose-1,6-bisphosphatase activity have been studied. At low (non-inhibitory) concentrations of fructose-1,6-bisphosphate K+ shows an inhibitory effect and the apparent Km for fructose-1,6-bisphosphate increases as the concentration of monovalent cation increases. The inhibition by high substrate concentrations is decreased by addition of the potassium ions. Modification of a highly reactive cysteine residue with cyanate or N-ethylmaleimide results in the loss of activation of the enzyme by K+. Significant protection to the loss of potassium activation and substrate inhibition is afforded by the presence of low concentrations of fructose-2,6-bisphosphate or inhibitory levels of fructose-1,6-bisphosphate. Non-inhibitory concentrations of the substrate give partial protection against the loss of monovalent cation activation. The inhibitor AMP markedly increases the reactivity of the cysteine residue. The carbamoylated enzyme is not inhibited by excess of Mg2+ as compared to native enzyme. The results suggest that K+ decreases the affinity of the enzyme for fructose-1,6-bisphosphate at both the catalytic site and an allosteric site for fructose-2,6-bisphosphate. Furthermore, they lead to the proposal that monovalent cations activation could be due to the removal of both Mg2+ and substrate inhibitions.